1. Field of the Invention
This invention relates to a scanning optical device and an image forming apparatus realized by using such a device. A scanning optical device according to the invention can suitably be used for an image forming apparatus such as a laser beam printer (LBP) or a digital copying machine.
2. Related Background Art
Scanning optical devices to be used for imageforming apparatus including laser beam printers and digital copying machines are adapted to cyclically deflect a light beam such as a laser beam that is optically modulated according to an image signal and emitted from a light source by means of an optical deflector such as a rotary polygon mirror, converge the deflected light beam to a spot of light on a surface of a photosensitive recording medium (photosensitive drum) by means of an imaging optical system having a so-called f.theta. feature and cause the light beam to scan the surface in order to record an image.
The optical requirements to be met by such a scanning optical system include that it can excellently focus the light beam on the photosensitive drum and it shows an f.theta. feature in the main-scanning direction. Japanese Patent Application Laid-Open No. 9-230274 proposes a scanning optical system satisfying those requirements. FIG. 1 of the accompanying drawings is a schematic perspective view of a principal portion of such a known typical optical scanner as applied to an image forming apparatus such as a laser beam printer or a digital copying machine.
Referring to FIG. 1, the light beam emitted from a semiconductor laser 21 is substantially collimated by a collimator lens 22 and then transformed into a divergent light beam by a spherical lens 46 having negative power. Then, the divergent light beam is made to enter a cylindrical lens 24 by way of a first fold mirror 18 to be converged in the sub-scanning section, i.e. a plane intersecting the optical axis along the sub-scanning direction. The converged light beam is reflected by a second fold mirror 25 and transmitted through an f.theta. lens system 27 having a spherical lens 27a and a toric lens 27b before it strikes the deflection surface (reflection surface) 26A of an optical deflector 26 and becomes focussed to form a substantially linear image (extending in the main-scanning direction) near the deflection surface. Note that the light beam striking the deflection surface 26A is made to show a predetermined angle relative to a plane (in which the optical deflector rotates and which is) perpendicular to the axis of rotation of the optical deflector in the sub-scanning section containing the axis of rotation of the optical deflector 26 and the optical axis of the f.theta. lens system 27.
On the other hand, the light beam entering the cylindrical lens 24 is not modified in the main-scanning section, i.e. a plane intersecting the optical axis along the main-scanning direction, and is then reflected by the second fold mirror 25 and transmitted through the f.theta. lens system 27 before it strikes the deflection surface 26A of the optical deflector 26 substantially along the center line of the deflection angle of the optical deflector 26 (front incidence). At this time, the divergent light beam is made to show a sufficiently large width relative to the facet width of the deflection surface 26A of the optical deflector 26 in the main-scanning direction by the collimator lens 22 and the spherical lens 46. Such an optical system is referred to as overfilled optical system.
The light beam deflected/reflected by the deflection surface 26A of the optical deflector 26 is led to the surface 31 of the photosensitive drum by way of the f.theta. lens system 27, a plane mirror 28 and a cylindrical mirror 34 having predetermined power only in the sub-scanning direction. Then, the light beam optically scans the surface 31 of the photosensitive drum in the direction of arrow B (main-scanning direction) as the optical deflector 26 is driven to rotate in the direction of arrow A. As a result of this scanning operation, an image is recorded on the surface 31 of the photosensitive drum operating as recording medium.
Referring to FIG. 1, an anti-dust glass panel 30 is arranged between the cylindrical mirror 34 and the surface 31 of the photosensitive drum to prevent fine particles of toner and paper floating in air near the surface 31 of the photosensitive drum from colliding with and adhering to the optical elements (on the rotary polygon mirror side).
A scanning optical device comprising an overfilled optical system and having a configuration as described above shows an excellent deflection efficiency due to its optical deflector and hence is adapted to high speed scanning operation. On the other hand, however, the quantity of light getting to a unit area of the surface to be scanned (surface of the photosensitive drum) varies between on axis and off axis in the main-scanning direction to make it impossible to realize a uniform distribution of quantity of light in the main-scanning direction. This is because the deflection surface having a width smaller than that of the light beam moves in the light beam with a changing deflection angle along the main-scanning direction. More specifically, the deflection surface is located at a position squarely facing the light beam when the latter is scanning an area on and near the optical axis, whereas it is located at a position inclined relative to the light beam when the latter is scanning an off-axis area. Thus, the ratio of the quantity of light striking the deflection surface to the total quantity of light emitted from the optical system for incident light of the device and hence the quantity of light getting to a unit area of the surface to be scanned vary as a function of the angle of the deflection surface.
Meanwhile, the intensity distribution of the light beam from a semiconductor laser is normally a Gaussian distribution, where the intensity of light is higher at the center than at the periphery of the light beam. Therefore, the quantity of light getting to a unit area of the surface to be scanned (surface of the photosensitive drum) is greater on axis than off axis in the main-scanning direction to produce an uneven distribution of quantity of light.